Construction of indazolo[3,2-: A] isoquinolines via [3 + 2] cycloaddition of benzynes

Article abstract of DOI:10.1039/c9ob01878f

A [3 + 2] annulation protocol for the construction of N-substituted indazolo[3,2-a]isoquinolines starting from benzynes and C,N-cyclic azomethine imines was developed. A diverse range of highly functionalized products indazolo[3,2-a]isoquinolines featuring an indazole scaffold can be easily accessed via a one-step reaction under mild conditions, and they show good anti-proliferative activity on cancer cells.

Full text of DOI:10.1039/c9ob01878f

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Construction of indazolo[3,2-a]isoquinolines via
3 + 2] cycloaddition of benzynes†
[
Cite this: Org. Biomol. Chem., 2019,
7, 8963
1
Yue-Kun Li, Ming-Xin Cui, Feng Sha, * Qiong Li and Xin-Yan Wu
*
Received 27th August 2019,
Accepted 21st September 2019
DOI: 10.1039/c9ob01878f
rsc.li/obc
A [3 + 2] annulation protocol for the construction of N-substituted
Indazolo[3,2-a]isoquinoline compounds have been used in
7
indazolo[3,2-a]isoquinolines starting from benzynes and C,N- many fields, yet only a few studies on their synthetic strategy
cyclic azomethine imines was developed. A diverse range of highly have been reported in recent decades. In 1977, the Reddy
functionalized products indazolo[3,2-a]isoquinolines featuring an group reported that the treatment of 1-(2-nitrophenyl)isoqui-
8
9
indazole scaffold can be easily accessed via a one-step reaction nolines with triethylphosphite gave dihydroindazolo[3,2-a]iso-
under mild conditions, and they show good anti-proliferative quinolines. Subsequent Pt/C hydrogenation furnished inda-
activity on cancer cells.
zolo[3,2-a]isoquinolines in 62–66% yields (Scheme 2a). A
1
0
similar result was observed by Stanforth in 1987. In 2005,
Kuo and co-workers reported the reaction of 1-(2-nitrophenyl)
isoquinolines with SnCl ·2H O, resulting in indazolo[3,2-a]iso-
2
2
1
1
quinolines.
Up to 77% yield was achieved, yet poor
Introduction
selectivity was demonstrated (Scheme 2b). Then György Hajós
Benzophenanthridine alkaloids are important bioactive reported a multistep approach to obtain fluorescent indazolo
natural products. Most of them are known to exhibit potent [3,2-a]isoquinoline-6-amines under microwave irradiation
1
12
anti-tumor, anti-inflammatory and bacteriostatic activities. In (Scheme 2c). On the basis of previous research studies,
recent years, many research studies have been carried out on exploration of synthetic strategies to obtain a series of structu-
the structure–activity relationship of various substituted ben- rally diverse indazolo[3,2-a]isoquinolines with higher yields
2
zophenanthridines. Indazolo isoquinoline derivatives have a and better chemoselectivity is of great interest.
1
3
similar ring system to benzophenanthridine alkaloids, and
exhibit diverse biological activities, as shown in previous litera- particularly those which can be generated from the corres-
Ever since the discovery of benzynes in the mid-1950s,
3
14
ture. For example, indazolo[3,2-a]isoquinolines have been ponding o-(trimethylsilyl)aryl triflates, they have emerged as
15
recognized as an effective DNA intercalator and exhibit anti- powerful synthons in organic synthesis. The cycloaddition of
4
cancer activities.
Moreover, indazolo[3,2-b]isoquinolines benzynes with diazo compounds is an attractive route for the
1
6
(
Scheme 1) have been found to inhibit the reverse transcrip- construction of the corresponding heterocyclic compounds.
tase activity of the Moloney leukaemia enzyme and RNA poly- Therefore, on the basis of our previous work on benzyne chem-
5
17
merase. It has been disclosed that indazolo[4,3-gh]isoquinoli- istry, we herein describe a convenient method of using benzyne
nones (Scheme 1) exhibit cytotoxic/anti-proliferative activity and C,N-cyclic azomethine imines to construct indazolo[3,2-a]iso-
against different cancer cell lines, namely KB/HeLa (cervical quinolines via [3 + 2] cycloaddition (Scheme 2d).
carcinoma), SKOV-3 (ovarian carcinoma), and SF-268 (CNS,
6
glioma).
School of Chemistry & Molecular Engineering, East China University of Science and
Technology, 130 Meilong Road, Shanghai 200237, P. R. China.
E-mail: shaf@ecust.edu.cn, xinyanwu@ecust.edu.cn; Tel: +86-21-64252011
†
Electronic supplementary information (ESI) available: Detailed experimental
procedures, characterization data, copies of NMR spectra for all previously
1
1
unknown products, and 2D H– H NOESY spectra of 3l, 4b, 4d and 4f. See DOI:
0.1039/c9ob01878f
1
Scheme 1 Examples of indazolo isoquinoline derivatives.
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a
Table 1 Screening optimal conditions
Entry 2a (equiv.) Time (h) T (°C) F−/solvent
3a yield (%)
b
1
2
3
4
5
6
7
8
1.2
2.0
3.0
4.0
3.0
3.0
3.0
3.0
5
5
5
5
5
5
5.5
4
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
0
TBAF/THF
TBAF/THF
TBAF/THF
TBAF/THF
36
42
91
90
c
KF/18-C-6/THF 46
c
CsF/MeCN
TBAF/THF
TBAF/THF
57
85
88
40
a
Reaction conditions: 1a (0.2 mmol, 1.0 equiv.), F source (0.8 mmol,
.0 equiv.), and solvent (1.2 mL). Isolated yield. 2.0 mL of solvent
b
c
4
was used. r.t. = room temperature.
With the optimal conditions in hand (Table 1, entry 3), we
then explored the generality of this [3 + 2] cycloaddition reac-
tion. As shown in Table 2, various C,N-cyclic azomethine
imines were prepared and reacted with benzyne precursors,
furnishing the desired products (3a–3l) in moderate to good
yields. As for azomethine imine 1e with the methyl group at
the 8-position, only 9% of 3e (Table 2) was isolated, even with
a prolonged reaction time. We speculated that it was probably
due to the methyl steric hindrance preventing the reaction
partners from coming together. Besides, comparing the effects
of different substituents on the 7-position, the methyl substitu-
ent of azomethine imine 1b was found to give the corres-
ponding product 3b (Table 2) in 87% yield. The halogen sub-
stituted C,N-cyclic azomethine imines also afforded the corres-
ponding compounds in good yields (Table 2, 3f–i). Therefore,
Scheme 2 Approaches a–d to synthesize indozolo[3,2-a]isoquinolines.
Results and discussion
Firstly, the reaction of C,N-cyclic azomethine imines 1a with the reaction of chloride- and bromide-substituted C,N-cyclic
1
8
the benzyne precursor 2a was selected as the model reaction.
azomethine imines (1f and 1g) furnished higher yields of the
As shown in Table 1, the target product indazolo[3,2-a]isoqui- corresponding products (3f and 3g). In addition, azomethine
noline (3a) was generated when 1.2 or 2.0 equivalents of the imine 1j with a naphthalenyl moiety could also be smoothly
benzyne precursor were used and TBAF was utilized as the fluor- converted into the corresponding product 3j with satisfactory
ide source (Table 1, entries 1 and 2). It was observed that the yield (71%). Also, the reaction of 4,5-dimethyl benzyne precur-
amount of 2a used affected the yield of 3a. To our delight, the sor 2b with C,N-cyclic azomethine imine 1a was studied. The
yield of 3a was improved obviously to 91% when the amount of corresponding indazolo[3,2-a]isoquinoline product 3k was suc-
benzyne precursor 2a was increased to 3.0 equivalents (Table 1, cessfully isolated in 82% yield.
entry 3). Further increase of the loading of 2a to 4.0 equivalents
led to no obvious change in the yield of 3a (Table 1, entry 4). compounds for anti-cancer activities, the debenzoylation
Meanwhile, two other fluoride sources KF/18-C-6 and CsF were reaction of 3a was further explored in the presence of LiBHEt
Considering that indazolo isoquinolines are important
1
9
3
tested, and they both gave 3a in moderate yields (Table 1, (Scheme 3). Gratifyingly, 5,6-dihydroindazolo[3,2-a]isoquino-
entries 5 and 6). It is presumed that the release rate of benzyne lines 4a were obtained in a perfect yield of 90%.
did not match well with the reaction under these conditions.
To disclose the regioselectivity of this [3 + 2] cycloaddition,
Then the effect of temperature on the reaction was investigated. nonsymmetric benzyne precursors were used. As expected, the
Product 3a was isolated in 85% yield at 0 °C (Table 1, entry 7), reaction of 2c with C,N-cyclic azomethine imine 1a afforded
and the reaction carried out at 40 °C gave 88% of 3a (Table 1, regioisomers 3l and 3l′ in a 1 : 1.2 ratio (Scheme 4, eqn (1)).
entry 8). Logically, the conditions described in entry 3 (Table 1) The structure of product 3l was confirmed by the NOESY
were chosen as the suitable reaction conditions.
study, which indicated that the methyl group was on the same
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Table 2 Reaction scope of C,N-cyclic azomethine imines and
a
benzynes
Scheme 4 Reaction scope of nonsymmetric benzyne precursors 2c
and 2d.
Scheme 5 The electronic effect of the reaction of 1a and 2d.
a
Reaction conditions: 1a–1j (0.2 mmol, 1.0 equiv.), 2a–2b (0.6 mmol,
3
.0 equiv.), TBAF (0.8 mmol, 4.0 equiv.), and THF (1.2 mL), at room
b
temperature, under a nitrogen atmosphere. Isolated yield.
Scheme 6 Reaction scope of nonsymmetric benzyne precursor 2e and
the NOESY study of the Heck coupling product 4c.
Scheme 3 Debenzoylation of compound 3a.
side of the isoquinoline moiety. To our surprise, the reaction reaction of the debenzoylated product 4c with methyl acrylate
of 3-methoxyl benzyne precursor 2d gave the sole regioisomer was subsequently conducted, so that the structure of product
3
m (56%, Scheme 4, eqn (2)). The observed excellent regio- 4d could be studied by the NOESY analysis (Scheme 6,
selectivity may be attributed to the electron induced effect of eqn (4)). The NOESY analysis of 4d showed that the β-H of
2
0
the methoxyl group. As demonstrated in Scheme 5, when 1a olefin hydrogen was related to the triplet of the phenyl ring.
came close to the o-methoxy benzyne in the reaction, the iso- Therefore, it is confirmed that the bromine substituent of 3n
quinoline moiety and the methoxyl group on the same side is on the same side of the isoquinoline moiety.
were favoured. After debenzoylation of 3m under LiBHEt
ditions, the structure of product 4b could be confirmed by the used and it successfully afforded the sole regioisomer product
NOESY study (Scheme 4, eqn (2)). 3o in 32% yield (Scheme 7, eqn (5)). This may be attributed to
3
con-
Furthermore, the 1,2-naphthalyne precursor 2f was also
It is interesting that the sole regioisomer 3n was obtained the higher steric hindrance of the naphthalene ring.
in 43% yield with the utilization of the o-bromo substituted Debenzoylation of 3o followed by the reaction with methyl tri-
benzyne precursor 2e (Scheme 6, eqn (3)). The Heck coupling fluoro-methanesulfonate afforded N-methyl triflate 4f. The
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Fig. 1 Cell toxicity of 3p on NCI-H460 and HCT-116.
Scheme 7 Reaction scope of nonsymmetric benzyne precursor 2f and
the NOESY study of product 4f.
NOESY study of 4f indicated that the naphthalene ring was on
the same side of the isoquinoline moiety (Scheme 7, eqn (6)).
To further explore the [3 + 2] cycloaddition, it is proposed
that the addition of the conjugated structure may also yield
the corresponding product under the same conditions. Thus,
isoquinolinium imide 1k was employed to react with the
benzyne precursor 2a. The reaction proceeded smoothly under
the optimal conditions and afforded the target product 3p in
excellent yield (92%, Scheme 8).
Fig. 2 Cell toxicity of 4a on NCI-H460 and HCT-116.
After obtaining a series of indazolo[3,2-a]isoquinolines (3
and 4), we mainly investigated their anti-proliferative activities
6
on human NSCLC NCI-H460 and colon carcinoma cells
1
7c
HCT-116
using the MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-
2
1
diphenyltetrazolium bromide) assay. As shown in Table 3,
the anti-proliferative activities of 3a, 3p and 4a were demon-
strated. The concentration of the compound that inhibited Fig. 3 Cell toxicity of 3a on NCI-H460 and HCT-116.
5
0% (IC50) of cell viability after 48 h was calculated by non-
2
2
linear regression (GraphPad Prism™). Compound 3p showed
good inhibitory activity on NCI-H460 (IC50 = 163.5 μmol L− ),
but weak cytotoxicity on HCT-116. The apoptosis rate of
NCI-H460 was fast when the concentration of compound 3p
was between 0 and 100 μmol L (Fig. 1). It was interesting to
observe that 4a exhibited excellent anti-proliferative activities
1
on two kinds of cancer cells, and showed a better effect on
−1
HCT-116 (IC50 = 126.9 μmol L ). Both cancer cell lines
were almost dead when the concentration of 4a reached
−
1
−1
4
00 μmol L (Fig. 2). We speculated that the structural change
of the dihydroisoquinoline moiety might be responsible for
the increased bioactivity. Compound 3a was found to show
weak inhibitory activity on both the cancer cell lines (Fig. 3).
Conclusions
In summary, we have developed a benzyne-based strategy for
the synthesis of indazolo[3,2-a]isoquinolines. Via this one-step
approach, a diverse range of highly functionalized isoquino-
lines featuring an indazole scaffold can be easily obtained in
moderate to good yields with high regioselectivity under mild
conditions. Furthermore, debenzoylation reaction could
smoothly furnish indazolo isoquinolines in excellent yields.
Scheme 8 Reaction of N-benzoyl isoquinolinium imide 1l with the
benzyne precursor 2a.
Table 3 Anti-proliferative activities on NCI-H460 and HCT-116 (IC50/μM)
Compound
NCI-H460
HCT-116 Good proliferative activities were observed towards cancer cell
lines NCI-H460 and HCT-116. This novel synthetic approach
provides an efficient access to indazolo[3,2-a]isoquinoline
derivatives with biological activity.
3a
3p
4a
412.9
163.5
218.9
1476
1463
126.9
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activity is based on the MTT assay. The cells were seeded in
the respective growth medium recommended by the sup-
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2
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8968 | Org. Biomol. Chem., 2019, 17, 8963–8968
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